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[SOLVED] Low Level AM Transmitter

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mr bobo

Newbie level 3
Nov 24, 2012
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Hello experts!

I am newbie here.

I am in the first year of Electronics Engineering. So my Analog Communication teacher gave us an assignment and told us to make the Low Level AM Transmitter. The assignment summary goes as follows

Project summary
The aim is to design, build and test an AM system using a carrier frequency in the
15-25kHz range. The modulating input signal will be an audio waveform in the
range 250Hz – 1.5kHz. The system will consist of a transmitter (modulator) and
receiver (demodulator) along with input and output filters and amplifiers as required.
Only two DC supply voltages are to be used and amplifiers/buffers should be opamp
based (e.g.TL071). A maximum of 4 op-amps can be used in the system
Transmitter: The transmitter will take an input sinusoidal signal with an amplitude of
no more than 200mV and a maximum frequency of 2kHz which will be used to
modulate the amplitude of a sinusoidal carrier wave in the 10-20kHz range. The
carrier frequency will be taken from the signal generator and you should build an
oscillator (e.g. Wien bridge) to provide the modulating signal.
Amplitude modulation should be achieved using a switching modulator based on a
nonlinear device: the performance of switching modulator circuits based on a diode
and a BJT should be compared.
Transmission line: In a real system a transmission line would typically have 50Ohm
impedance. In the project the transmission line will be replaced with a 50Ohm
resistor to provide the impedance between transmitter and receiver.
Receiver: The receiver will be based on an envelope detector circuit with additional
filtering as required to give an output signal which is a replica of the input
modulating signal. The output from the receiver should AC coupled with an
amplitude of at least 200mV when driving a 50Ohm load.

He went on to give me a specific signal frequency of 420hz and the carrier frequency of 21Khz

i have managed to look around on the web for some information and i did background research and designed schematic using isis but i have hit the wall beacuse im so confused on how to go further. I have attached a pic of my schematic

i would appreciate it if any one of you expert will help me because i only got 4days to finish the design and start on the testing of the it


Thank you.

Following your schematic and drawing it in a simulator, I have the first stage producing square waves.

The second stage rounds them to a reasonable sine shape.

I found that a few components can be omitted.

Are you able to obtain a sine wave output right after R11?

Have you located a schematic showing how to make a Wien-bridge oscillator, to provide the 420 Hz signal?

And a schematic showing how to use it to modulate the carrier?

The first stage is giving me a sine wave not a square wave and the second stage is just giving me a straight line.

After R11 im still getting a staright line :-( i do not know where im going wrong.

i have not located a good schematic on how to make a good Wein-bridge oscillator to provide a 420Hz signal.

Which other components can be ommited?

Is it possible i can have a look at the schematic you are working from and try try and get my head around it.:???:

I guess it's all right to show you the layout that works for me, since it started with your schematic.

I made this using an animated simulator found (free) at:

The op amps need the right volt levels at the inputs, in order for the waveform to swing between the supply rails.

- - - Updated - - -

I used a larger capacitor, which brings the frequency of oscillation down a bit, because the simulator often gets quirks above 20 or 25 kHz.

Thank you very much Bradtherad

i have removed some of the components and my circuit is the same as the the one you have simulated by for some weird reasons the first satge is giving me a sine wave, as in the picture below.
stage 1.png

And the 2nd stage is giving me a straight line as in the picture below
Stage 2.png

Is it possible you can point me in the right direction in what im doing wrong

And i really appreciate the help you are giving me, you are a life man


Looking back at your OP...

The carrier frequency will be taken from the signal generator

It appears your assignment permits you to bring in a 21 kHz carrier signal from an external sinewave generator. (I had believed you were constructing your own.)

Question: Are you building only the 420 Hz oscillator? Or both oscillators?


The assignment mentions a Wien bridge oscillator.

you should build an oscillator (e.g. Wien bridge) to provide the modulating signal.

Question: Is this a suggestion? Or is it required that you use a Wien bridge type?


Stage 1 of your schematic appears intended to generate the carrier waveform (the higher frequency).

Perhaps I got the wrong idea? My simulation showed it generating DC pulses. Therefore I got the impression your stage 2 is intended as a filter to convert the squarish waves to sine waves. Yet you report that you do not get the same waveforms in your simulation.

Question: Please clarify as to what is the function of your schematic? This must be defined in order to go to the next step in developing the project.

You still need a schematic which modulates the carrier.


Op amps can be powered from a single supply or a bipolar supply. For the time being during the development stage, it is easier to use a bipolar supply.

There is also the characteristic of a 741 (popular op amp) that its output cannot go within 2V of the more negative supply rail.

Question: Can you set certain parameters for the op amp, which cause it to behave like a 741, or a 324, etc.?

Question: Is their output range determined by the volt levels which are applied to pins 4 and 7 in your schematic?

Thank you very much Bradtherad, i have managed to see the errors i was doing, i really appreciate all the effort and time you put to resolve my problem. thank man you are a life saver

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